1. Technical Field
The invention relates to network interfaces for asynchronous transfer mode networks which multiplex and demultiplex isochronous and non-isochronous data streams on a single physical channel, independent of host processors.
2. Description Of Related Art
Isochronous and non-isochronous information have different characteristics which impose different demands on communication networks. As a result, different transfer modes were developed to transport information from one point to another along particular communication networks.
For the present application non-isochronous data will also be referred to as bursty data. Such bursty type data includes for example, file transactions and e-mail transactions. Isochronous data, on the other hand, is often transferred between end points using a circuit switching transfer mode which establishes and maintains a continuous circuit between the end points for the duration of the communications.
Typically, bursty data has a high peak-to-average rate ratio, mandates error-free transmission, and has relatively course end-to-end timing constraints. For example, a remote database access may specify that a query message be delivered from client to a server in under 5 seconds 95 percent of the time. However, if a transmission error is detected, the correct response is to retransmit the query message even if the 5 second specification is violated. Traditional local area networks (LANs), such as Ethernet and Token Ring networks, are designed to transfer bursty data.
In contrast, isochronous data is regular in time, i.e., has a fixed bandwidth, and usually does not require error-free transmission since errors typically result as audio/video noise. In addition, isochronous data has stringent end-to-end timing constraints. Traditional wide area networks (WANs) like the telephone network and integrated services digital networks (ISDN's) are geared for isochronous data. These networks can support composite streams with both isochronous and bursty data traffic, however they are heavily biased toward isochronous traffic. For example, an ISDN 128 kbps Basic Rate Interface (BRI) is a reasonable medium for the voice and video components of an integrated voice/video/data call, but the data rate of the data component is likely 64 kbps or less, which is substantially slower than the 10 Mbps data rates realized on traditional local area networks.
Traditional local area networks and wide area networks are not well suited to transfer composite data streams of isochronous and non-isochronous information. In general, local area network technology (both hardware and software) is intrinsically biased for bursty data. Multiplexing multiple logical connections (with potentially different data types) onto the network is an upper-layer software construct, adapted for time-insensitive, error-free data connections. However, isochronous, e.g., voice and/or video, information transmission requires a guaranteed bandwidth and tightly bounded delivery delays. Low-end isochronous streams may be viable over traditional LANs, however the resolution and quality achieved is often unacceptable for practical uses. Moreover, as the demands imposed on communication networks increase, the transfer of isochronous data along traditional local area networks is impracticable. For example, isochronous information streams require real-time processing which cannot be provided by current local area networks and host processors, except when transporting information streams with low bandwidths.
Wide area network technology has evolved to provide isochronous and bursty data stream transmission capabilities. Such wide area networks designs are primarily directed to transfer isochronous data independent of host processors and secondarily to transfer bursty data at rates of approximately 64 kbps. However, such bursty data rates are unacceptable for multimedia communications, which require bursty data to be transmitted at a rate of at least 10 Mbps. An example of the above described wide area network is the NCR 3336 Telemedia Connection (TMC) which has a network feed (ISDN BRI) connected directly into an isochronous stream processor (e.g., an AVP Video CODEC chipset, manufactured by AT&T). The stream processor multiplexes and demultiplexes the composite audio/video/data stream, diverting the isochronous components to the appropriate real-time processors, and forwarding the bursty data component to appropriate host processors.
An emerging transfer mode technology developed to overcome the drawbacks of conventional LAN and WAN technologies in network communications is the asynchronous transfer mode (ATM) technology in, for example, Broadband Integrated Services Digital Network (B-ISDN) which is designed to transfer voice, video, image and data. Transfer modes, such as message switching, circuit switching and packet switching, used with the above noted network technologies are primarily dedicated for either isochronous or non-isochronous data and are not adaptable for efficient use in emerging voice, video and data communication networks, such as B-ISDN technology. Thus, asynchronous transfer mode technology was developed to provide communication networks with the capability to support the different kinds of data traffic.
Generally, the ATM technology supports four classes of traffic: 1) constant bit rate, connection-oriented, synchronous traffic (e.g., uncompressed voice and video data); 2) variable bit rate, connection-oriented, synchronous traffic (e.g., compressed voice and video data); 3) variable bit rate, connection-oriented, asynchronous traffic (e.g., X.25); or 4) connectionless packet data (e.g., local area network traffic). ATM technology is based on a cell switching technique which packages or organizes information, such as isochronous or non-isochronous data, into fixed length packets called cells. Each ATM cell is 53 bytes long and includes 5 bytes for a header and 48 bytes for a payload. The ATM header includes various fields used to ensure efficient and accurate transfers of data along the network. One such field is the virtual channel identifier (VCI) which is used to identify the virtual channel upon which the cell is to be transferred.
Asynchronous transfer mode technology is a connection-oriented technology wherein host processors desiring to communicate are required to establish and maintain a logical or virtual connection therebetween. ATM technology utilizes a three layer architecture to facilitate isochronous and non-isochronous data communications. The three layers are the Physical layer, the ATM layer and the ATM Adaption layer (AAL). However, current ATM technology requires traditional host processors to establish and maintain communication links and to route the different data streams (e.g., bursty or isochronous) over the proper communication link. As a result, the number of CPU cycles allocated for data applications in the host processor is reduced in order to accommodate the network communication tasks.
Moreover, traditional host processors are not designed to process real-time isochronous data. As a result, the processing of the isochronous data imposes a constant bandwidth restriction on the host processor, which also reduces the number of CPU cycles allocated for data applications. FIG. 1 illustrates a prior art ATM network architecture 10 having isochronous processors 12 connected to host data processors 14 and the host processors connected to known ATM switches 16. A broadband integrated services digital network (B-ISDN) 18 utilizing ATM technology resides between each ATM switch.
Based upon the above-described drawbacks with current ATM technology, a need exists for an ATM network interface which is capable of multiplexing and demultiplexing composite data streams onto a single logical connection independent of a host processor, and which is capable of isolating isochronous data from the host processors so as to reduce the number of CPU cycles dedicated by the host processor to network communications.